Electroanalytical chemistry encompasses a group of quantitative analytical methods that are based upon the electrical properties of an electrolyte solution when it is made part of an electrochemical cell. Electrochemical methods make possible the determination of the concentration of each of the species in the electrolyte solution. Dynamic interfacial methods, in which currents in electrochemical cell play a vital part, are of several types. In controlled-potential methods, the potential of the cell is controlled while measurements of other variables are carried out. Generally, these methods are considered to be sensitive with respect to other methods in the electroanalytical domain, and have relatively wide theoretical ranges which typically does not go lower than 0.5 ppb. Furthermore, many of these procedures can be carried out with microliter volumes of sample. Thus, detection limits in the ppb range can be realized.
Voltammetry and polarography comprise a group of electroanalytical methods in which information about the analyte is derived from the measurement of current as a function of applied potential obtained under conditions that encourage polarization of an indicator, or working, electrode. Generally, working electrodes in voltammetry have a surface area of about a square millimeter, and in some applications, in order to enhance the limiting current, microelectrodes of a few square micrometers (and even less) are used individually or assembled in arrays. In voltammetry, a variable potential excitation signal is impressed by a potentiostat upon an electrochemical cell containing a working electrode. This excitation signal elicits a characteristic current response upon which the method is based. The waveforms of the most common excitation signals used in voltammetry are: the linear scan wave (in polarography hydrodynamic voltammetry), the differential pulse wave (in differential pulse voltammetry), the square wave (in square wave voltammetry), and the triangular wave (in cyclic voltammetry).
In each case, the electrochemical cell represents an electrical equivalent circuit defined in terms of RC components models according to the type of reaction taking place in the cell. The fundamental parameters are the applied potential E, the resulting current i, and the time t. E has a thermodynamic effect on the equilibriums or on the reactional speed, e.g. Nerst relation for the simple cases. The elementary action resides in the application of a step potential perturbation, after which the resulting current response is sampled. The various implemented methods, modes, variants are distinctive only in regard with the arrangements and protocols of imposition of these elementary perturbations repeated together with a change in the base potential. Historically, initial polarography had a sensitivity limit of approx. 1 ppm. Normal pulse polarography (NPP) pushed back the sensitivity limit to approx. 0.1 ppm. At its turn, differential pulse polarography (DPP) pushed back the sensitivity limit to approx. 10 ppb. Square wave voltammetry (SWV) further pushed back the sensitivity limit to approx. 1 ppb. A variation of the DPP method which uses both edges of the perturbation signal, improved the sensitivity by a factor of two, i.e. approx. to 0.5 ppb, whereas multiple square wave voltammetry (MSWV) and differential integrated multiple pulse polarography (DIMPP) achieved a sensitivity of approx. 0.1 ppb.
Known in the art is Canadian patent No. 1.227.244 (Eccles et al.), which describes a method and an apparatus for electroanalytic purposes based on DPP to identify the nature or concentration of species. U.S. Pat. No. 5,186,798 (Sakai et al.) describes a device and a method for performing electrolysis in multiple pulse mode, such as NPP, SWV, DPP and reverse pulse polarography (RPP).
The articles entitled: "Triple-Pulse Voltammetry and Polarography", Serna et al., Anal. Chem. 1993, 65, pp. 215-222; "Double Differential Pulse Voltammetry", Camacho et al., Journal of Electroanalytical Chemistry, 365 (1994), pp. 97-105; and "General Analytical Solution for a Reversible i-t Response to a Triple Potential Step at an SMDE in the Absence/Presence of Amalgamation", Molina et al., Journal of Electroanalytical Chemistry 408 (1996) pp. 33-45, describe theoretical and analytical aspects of a voltammetric method based on the use of a triple pulse of potentials.
Other devices and methods are described in U.S. Pat. No. 4,628,463 (Sturrock et al.), U.S. Pat. No. 5,192,403 (Chang et al.), U.S. Pat. No. 4,917,774 (Fisher), U.S. Pat. No. 5,104,809 (Moulton), U.S. Pat. No. 5,169,510 (Lunte et al.), U.S. Pat. No. 5,217,112 (Almon), U.S. Pat. No. 4,302,314 (Golimowski et al.), U.S. Pat. No. 4,805,624 (Yao et al.), U.S. Pat. No. 5,196,096 (Chang et al.), U.S. Pat. No. 4,767,994 (Hopkins et al.), U.S. Pat. No. 4,058,446 (Zirino et al.), U.S. Pat. No. 4,059,406 (Fleet), European patent application published under No. 0.343.702 (Guerriero et al.) on 29.11.89, German patent application published under No. 2.451.659 (Bednarski et al.) on 28.5.75, and the articles entitled "Theory of multiple square wave voltammeters", Fatouros et al., J. Electroanal. Chem., 213 (1986), pp. 1-16, "Multiple square wave voltammetry: experimental verification of the theory", Krulic et al., J. Electroanal. Chem., 287 (1990), pp. 215-227, and "Electrochemical mercury detection", Turyan et al., Nature, Vol. 362, Apr. 22, 1993, pp. 703-704. However, these devices and methods have sensitivity limits lower than 0.1 ppb or, like in the case of the method reported by Turyan et al. in the above cited article, involve electrochemical regeneration of the electrodes after each experiment and are highly selective (detection specialized for a single species) and thus not flexible. This last case is exemplary of the desirability of a flexible, efficient and reliable method and an instrument which can detect the concentration levels of the traces and ultra-traces of species in the environment, i.e. 1-10 ppt.
Many factors can influence the validity and accuracy of the results. Since the measurements derive from an excitation signal producing a potential difference in the electrochemical cell, the cleaner the excitation signal, the better the results. The command signal that controls the potentiostat applying the excitation signal to the electrodes of the cell must have a high accuracy and be noise free to not alter the electrochemical measurement of the faradic current. However, the prior art devices and methods lack for signal generators producing high accuracy and noise free command signals.
The imposition of a potential step to the electrodes in an electrochemical cell produces a current flow between these electrodes. This current, which is the current to be measured, has essentially two main components: a capacitive component and a faradic component. The capacitive current corresponds to the current required to charge the electric double layer at the interface of the working electrode and the solution (voltammetry is in the group of interfacial methods). The electric double layer is inherent to interfacial electrochemical phenomena and acts in terms of electronic components like an electric capacitor. This capacitance is important (in the order of 10 .mu.F/cm.sup.2 ) because of its small dielectric thickness. The faradic current corresponds to the current produced by the electrochemical reactions of oxido-reduction that occur at the working electrode. This faradic current is generally governed by the diffusion of electroactive species inside the gradient of concentration caused by the applied potential at the working electrode. The faradic current is very small, up to many orders of magnitude smaller than the capacitive current, when the concentration of the species in solution to be measured is at the level of traces or ultra-traces. The equivalent electronic component is an electric resistor whose value is a function of the concentration of the species to be measured. The equivalent electric circuit that represents the arrangement of these two components is a circuit where the capacitor and the resistor are connected in parallel. After the imposition of the potential step, the decreasing of the capacitive current is fast and is produced as a function of exp.sup.(-t/RC) whereas the decreasing of the faradic current is slow and is produced as a function of t.sup.(-1/2) which denotes the diffusion control. The main interest resides in the evaluation of the concentration of the species in solution, which is carried out by measuring the faradic current. The prior art methods consist in allowing a sufficiently long delay to elapse so that the capacitive current becomes small, before proceeding to the measurement of the faradic current. This delay however slows down the process.
Three electrodes apply a potential and measure the resulting current. They are in contact with the solution to be studied in the electrochemical cell. These electrodes are the working, reference and counter (or auxiliary) electrodes. The potential applied on the working electrode is the potential that the electrode/solution interface takes with respect to the environing solution. This applied potential refers to the potential of the reference electrode whose value must remain stable and fixed with respect to the solution, under some thermodynamic equilibrium. The applied potential must therefore be constrained to follow the value of the imposed potential, which amounts to a potential regulation. In the prior art apparatuses, the current measurement is carried out by the use of a current to voltage converter said to be at virtual ground, in the circuit of the working electrode. As a consequence, this has the effect of producing alterations and instabilities of the potential impressed on the working electrode during the current measurements.
To develop a current in an electrochemical cell, a driving force in the form of a potential is required to activate the reaction of ions at the interface of the solution and the working electrode. Just as in metallic conduction, this force follows Ohm's law and is equal to the product of the current in amperes and the local resistance of the solution in ohms. The force is generally referred to as the ohmic potential or the IR drop. In practice, the reference electrode refers to the potential of the bulk of the solution and not to the real potential at the interface where the reaction takes place. The net effect of the IR drop is to increase the potential required to operate an electrolytic cell. The effective potential of the electrode may drop dramatically in the presence of a current, and thus require correction to accelerate the charge of the double layer at the interface of the working electrode and the solution. This correction contributes to a correct double layer charging since the interface capacitance must be adjusted at the right potential and the corresponding component in the overall current is very important at short times. Presently, the correction is calculated according to theory, and may prove inadequate.
The current resulting from the electrodes under imposition of a potential step contains noise derived from various sources, for example the power supply of the instrument. It is therefore desirable to suppress as much as possible this noise, operation that the prior art devices and methods often fail to achieve properly.